The endocrine system is the body’s chemical messaging network. Unlike the nervous system, which sends electrical impulses along nerves at high speed for quick responses, the endocrine system uses hormones — chemical messengers secreted into the bloodstream — for slower, longer-lasting regulation of body functions.
Think of the nervous system as a text message and the endocrine system as a handwritten letter. Texts are fast but brief. Letters take longer to arrive, but they carry sustained instructions.
Every process from growth, reproduction, and metabolism to your response to stress and the regulation of blood sugar is coordinated by hormones. NEET dedicates significant weight to this chapter, and it deserves careful attention.
Key Terms and Definitions
Hormone: A chemical messenger secreted by an endocrine gland directly into the blood. Acts on target organs (cells with the correct receptor).
Endocrine gland: A ductless gland that secretes hormones directly into the bloodstream. Contrast with exocrine glands (which secrete into ducts, like salivary glands or the pancreas’s digestive enzymes).
Target organ: The organ or tissue that responds to a particular hormone. It has specific receptors for that hormone.
Feedback mechanism: A system where the output of a process controls the input. Negative feedback is most common — rising hormone levels signal the gland to reduce production.
Hypersecretion: Excess hormone production (e.g., Cushing’s syndrome from excess cortisol).
Hyposecretion: Insufficient hormone production (e.g., dwarfism from insufficient GH).
Major Endocrine Glands and Their Hormones
Hypothalamus (Master Regulator)
The hypothalamus is not just a brain region — it’s the link between the nervous and endocrine systems. It produces releasing and inhibiting hormones that control the pituitary gland.
Key hormones: GnRH (gonadotropin-releasing hormone), TRH (thyrotropin-releasing hormone), CRH (corticotropin-releasing hormone).
Pituitary Gland (Master Gland)
Located below the hypothalamus, the pituitary has two lobes:
Anterior lobe (Adenohypophysis):
- GH (Growth Hormone): Stimulates growth of long bones and muscles. Hyposecretion → dwarfism; hypersecretion → gigantism (in children) or acromegaly (in adults).
- TSH (Thyroid Stimulating Hormone): Stimulates thyroid to produce thyroxine.
- ACTH (Adrenocorticotropic Hormone): Stimulates adrenal cortex to produce cortisol.
- FSH (Follicle Stimulating Hormone): Stimulates ovarian follicle development (females) and sperm production (males).
- LH (Luteinizing Hormone): Triggers ovulation and testosterone production.
- Prolactin: Stimulates milk production.
Posterior lobe (Neurohypophysis): Stores and releases hormones made in the hypothalamus:
- ADH (Antidiuretic Hormone / Vasopressin): Promotes water reabsorption in kidneys. Deficiency → diabetes insipidus.
- Oxytocin: Triggers uterine contractions during labour; promotes milk let-down.
Thyroid Gland
Located in the neck. Produces:
- Thyroxine (T₄) and Triiodothyronine (T₃): Regulate basal metabolic rate (BMR), growth, and development.
- Hyposecretion in children → cretinism (stunted growth, mental retardation)
- Hyposecretion in adults → hypothyroidism (weight gain, lethargy), goitre if iodine deficient
- Hypersecretion → hyperthyroidism (Graves’ disease), weight loss, exophthalmos
- Calcitonin: Reduces blood calcium by promoting bone deposition.
Parathyroid Glands
Four small glands embedded in the thyroid. Produce:
- PTH (Parathyroid Hormone): Increases blood calcium by promoting bone resorption and calcium reabsorption in kidneys. Antagonist to calcitonin.
Adrenal Glands (Suprarenal Glands)
Sit on top of each kidney. Two parts:
Adrenal cortex (outer layer):
- Glucocorticoids (Cortisol): Regulate glucose metabolism, reduce inflammation. Excess → Cushing’s syndrome.
- Mineralocorticoids (Aldosterone): Regulate Na⁺ and K⁺ balance in kidneys. Excess → hypertension.
- Androgens (small amounts): Sex hormones.
Adrenal medulla (inner layer):
- Adrenaline (Epinephrine) and Noradrenaline (Norepinephrine): “Fight or flight” hormones. Released in response to stress/danger. Increase heart rate, blood pressure, blood glucose.
Pancreas (Islets of Langerhans)
Dual function organ — exocrine (digestive enzymes) and endocrine:
- Alpha cells: Secrete glucagon → raises blood glucose by stimulating glycogen breakdown (glycogenolysis) and gluconeogenesis.
- Beta cells: Secrete insulin → lowers blood glucose by promoting glucose uptake and glycogen synthesis.
- Delta cells: Secrete somatostatin (inhibits both insulin and glucagon release).
Diabetes mellitus: Insulin deficiency (Type 1) or insulin resistance (Type 2) → hyperglycaemia → glycosuria.
Gonads
Testes: Secrete testosterone (androgen) — male secondary sexual characters, spermatogenesis.
Ovaries: Secrete oestrogen and progesterone — female secondary characters, menstrual cycle regulation, pregnancy maintenance.
Thymus
Secretes thymosins — critical for T-lymphocyte maturation and immune function. Most active during childhood; degenerates after puberty.
Pineal Gland
Secretes melatonin — regulates circadian rhythms (sleep-wake cycle), body temperature, and reproductive cycles in some animals.
Feedback Mechanisms
Most endocrine regulation operates through negative feedback:
- Hypothalamus secretes TRH → Pituitary secretes TSH → Thyroid secretes thyroxine
- Rising thyroxine levels signal the hypothalamus and pituitary to reduce TRH and TSH secretion
- Thyroxine falls back to normal
This self-regulating loop maintains homeostasis. If the thyroid is removed (low thyroxine), the feedback signal is gone, and TSH stays permanently high — this is how TSH levels are used to diagnose hypothyroidism.
Positive feedback (rare) amplifies a signal: oxytocin during labour increases uterine contractions → more oxytocin → stronger contractions → delivery.
Exam-Specific Tips
NEET: This chapter is a guaranteed 3–5 questions every year. Focus areas: (1) specific hormone-disease pairs (GH deficiency → dwarfism), (2) the hypothalamus-pituitary axis, (3) insulin/glucagon in blood glucose regulation, (4) adrenal hormones in emergency response. Know all glands, their hormones, and the consequences of hypo/hypersecretion.
CBSE Class 10: Simpler treatment — adrenaline, insulin, thyroxine, and their roles. Know where each is produced.
CBSE Class 11: Detailed coverage — all glands, feedback mechanisms, and disease conditions. Diagram of human body with endocrine glands marked is frequently asked.
Common Mistakes to Avoid
Mistake 1: Confusing glucagon and insulin effects. Insulin lowers blood glucose (it’s released when glucose is high — after a meal). Glucagon raises blood glucose (released when glucose is low — between meals). They are antagonists.
Mistake 2: Saying pituitary is the “master gland” but forgetting hypothalamus controls it. The hypothalamus is the true master — it controls the pituitary through releasing/inhibiting hormones. The pituitary is the “master gland” only in the sense that it controls many other glands.
Mistake 3: Mixing up T₃ and T₄. T₄ (thyroxine) has 4 iodine atoms; T₃ has 3. T₃ is more potent. Both regulate BMR.
Mistake 4: Confusing adrenal cortex and medulla hormones. Cortex = steroid hormones (cortisol, aldosterone). Medulla = catecholamines (adrenaline, noradrenaline). “Fight or flight” = medulla. Inflammation control = cortex.
Mistake 5: Goitre = always hyperthyroidism. Goitre (enlarged thyroid) can occur in both hypothyroidism (especially iodine-deficiency goitre) and hyperthyroidism. It’s an enlargement, not a function descriptor.
Practice Questions
Q1: Which hormone is secreted in a “fight or flight” situation?
Adrenaline (epinephrine) and noradrenaline (norepinephrine), secreted by the adrenal medulla. They prepare the body for emergency response — increased heart rate, dilated pupils, redirected blood flow to muscles.
Q2: What is the effect of insulin deficiency?
Insulin deficiency → cells cannot take up glucose → blood glucose remains high (hyperglycaemia) → excess glucose excreted in urine (glycosuria) → diabetes mellitus. Over time, this damages blood vessels, kidneys, nerves, and eyes.
Q3: Differentiate between hypothyroidism and hyperthyroidism in adults.
Hypothyroidism (low thyroxine): reduced BMR, weight gain, lethargy, cold intolerance, goitre if iodine-deficient. Hyperthyroidism (high thyroxine): elevated BMR, weight loss, heat intolerance, nervousness, exophthalmos (in Graves’ disease).
Q4: Name the hormone responsible for lowering blood calcium levels.
Calcitonin, secreted by the thyroid gland. It promotes calcium deposition into bones, reducing blood calcium. Antagonised by PTH (parathyroid hormone) which raises blood calcium.
Q5: Why is the hypothalamus called the “neuroendocrine integrator”?
The hypothalamus receives neural signals from the brain (about stress, temperature, emotion) and converts them into hormonal signals (TRH, CRH, GnRH) that control the pituitary. It is the physical and functional bridge between the nervous system and the endocrine system.
FAQs
Q: Why are hormones effective in very small concentrations? Hormones bind to highly specific receptors on target cells. Even a few molecules can trigger a cascade of intracellular reactions (signal amplification) — each step multiplies the effect. This is similar to a lock-and-key mechanism where a tiny key opens a door that lets in a river of activity.
Q: Can a person produce too many hormones? Yes — this is called hypersecretion, usually due to a tumour in the gland. Cushing’s syndrome (excess cortisol), acromegaly (excess GH in adults), and hyperthyroidism are all examples. Treatment involves removing or ablating the overactive gland tissue.
Q: What is the difference between hormones and enzymes? Both are biological molecules that regulate processes. But enzymes are catalysts that act at the site of production and are not transported in blood. Hormones are messengers transported through blood to act at distant target organs. Also, enzymes are not “used up” in reactions; hormones are degraded after acting.
Additional Concepts
Hormone-disease pairs (NEET must-know table)
| Hormone | Hyposecretion | Hypersecretion |
|---|---|---|
| GH (child) | Dwarfism | Gigantism |
| GH (adult) | — | Acromegaly |
| Thyroxine (child) | Cretinism | — |
| Thyroxine (adult) | Myxoedema | Graves’ disease |
| Insulin | Diabetes mellitus | Hypoglycaemia |
| ADH | Diabetes insipidus | — |
| Cortisol | Addison’s disease | Cushing’s syndrome |
| Aldosterone | Low BP, high K | Conn’s syndrome |
NEET tests hormone-disease pairs almost every year. The most repeated: GH deficiency = dwarfism, insulin deficiency = diabetes mellitus, ADH deficiency = diabetes insipidus (dilute urine, excessive thirst).
Blood sugar regulation — a feedback loop
After a meal: blood glucose rises beta cells of islets of Langerhans release insulin insulin promotes glucose uptake by cells and glycogen synthesis in liver blood glucose falls.
Between meals: blood glucose drops alpha cells release glucagon glucagon promotes glycogenolysis and gluconeogenesis blood glucose rises.
This is negative feedback — the output (glucose level) controls the input (hormone release). Diabetes mellitus breaks this loop at the insulin step.
Emergency hormones — the fight-or-flight response
When threatened: hypothalamus sympathetic nervous system adrenal medulla releases adrenaline and noradrenaline.
Effects: heart rate increases, pupils dilate, bronchi dilate, blood diverted from gut to muscles, glycogen breaks down to glucose, mental alertness sharpens.
This is the fastest endocrine response because the adrenal medulla is essentially a modified ganglion — part nervous tissue, part gland.
Additional Practice Questions
Q6. Name the gland that secretes melatonin. What is its function?
Pineal gland. Melatonin regulates circadian rhythms (sleep-wake cycle) and body temperature. Secretion increases in darkness and decreases in light.
Q7. What is the role of thymosin?
Thymosin (from the thymus gland) promotes differentiation and maturation of T-lymphocytes, which are critical for cell-mediated immunity. The thymus is most active in childhood and degenerates after puberty.
Q8. Differentiate between diabetes mellitus and diabetes insipidus.
Diabetes mellitus: caused by insulin deficiency or resistance. High blood glucose, glucose in urine. Diabetes insipidus: caused by ADH deficiency. Normal blood glucose, but very dilute urine in large volumes. Both cause excessive thirst and urination, but for completely different reasons.